The present invention relates to sensors. More particularly, but without limitation, the present invention relates to sensors having an increased quality factor.
The advent of high-density plasma sources and fluorine-based etch chemistries, as well as developments in wet etching techniques, with high anisotropy and surface smoothness, have made it possible to realize quartz crystal shear-mode resonators with thicknesses less than 30 μm and diameters down to 100 μm1,2. These resonators can be easily configured as high sensitivity mass sensors, and are known as a quartz crystal microbalance (QCM)3. Until now, the large size of the QCMs has limited their widespread use for bio(chemical) sensing. Planar arrays of these mass sensors can now be realized, and without the drawbacks associated with flexural components4,5. These micromachined QCM arrays promise to be a robust platform for future (bio)chemical sensors.
The concept of mass measurement quartz resonators was first presented by Sauerbrey who found that the frequency change Δf is related to the mass loading Δm by6
                                          Δ            ⁢                                                  ⁢            f                    =                                    -                              (                                  2                  ⁢                                                                                    f                        0                        2                                            ⁡                                              (                        0                        )                                                              /                    A                                    ⁢                                                                                    ρ                        q                                            ⁢                                              μ                        q                                                                                            )                                      ⁢            Δ            ⁢                                                  ⁢            m                          ,                            (        1        )            where f0(0) is the unloaded resonant frequency, μq is the shear modulus, ρq is the density, and A is the area of the electrode on the quartz crystal7. The minus sign indicates the resonance frequency decreases upon mass loading. The relation given by eq. (1) holds only when the thin adsorbed film is well anchored to the sensor surface and not subject to viscous losses. A micromachined 30 μm thick and 11 mm diameter resonator is expected to have a sensitivity of ˜1 pg/Hz, a factor of ˜20,000 improvement in absolute mass sensitivity in comparison to a commercially available 5 MHz device3.
It is implicitly assumed that the high Q-factor of the resonators necessary to achieve high mass resolution is maintained to: achieve the improved mass sensitivity by miniaturization of the QCM. To this end, the energy loss mechanisms in the resonating quartz crystal need to be minimized, and the acoustic energy needs to be confined to within the active (electrode) area of the crystal. Due to the energy trapping effects, it been observed that the differential mass sensitivity of the QCM is a maximum at the center and decreases towards the edges of the electrode8,9. In fact, the simple proportionality found between the frequency shift and the deposited mass in eq. (1) is valid only if the material is homogeneously distributed over the crystal10. For example, on a 1 mm diameter resonator, a uniform gold film 7×10−5 nm thickness needs to be uniformly deposited to observe the predicted sensitivity of 1 pg/Hz. Thus, in spite of all the advances in the miniaturization of the quartz resonators, direct mass calibration curves following the Sauerbrey equation (eq. 1) have been reported only for nanogram loadings2. From these calibration curves, extremely high sensitivities are typically projected, i.e., picograms or even femtograms. However, they have not actually been directly demonstrated.